ORIGINAL RESEARCH published: 29 April 2015 doi: 10.3389/fmicb.2015.00353
Identification of signatory secondary metabolites during mycoparasitism of Rhizoctonia solani by Stachybotrys elegans Rony Chamoun † , Konstantinos A. Aliferis † and Suha Jabaji * Department of Plant Science, McGill University, Sainte-Anne-de-Bellevue, QC, Canada
Edited by: Helio K. Takahashi, Universidade Federal de São Paulo, Brazil Reviewed by: Marcelo Tolmasky, California State University Fullerton, USA Marcos Sergio Toledo, Universidade Federal de São Paulo, Brazil *Correspondence: Suha Jabaji, Department of Plant Science, McGill University, 21111 Lakeshore Rd., Sainte-Anne-de-Bellevue, QC H9X 3V9, Canada
[email protected] †
These authors have contributed equally to this work. Specialty section: This article was submitted to Fungi and Their Interactions, a section of the journal Frontiers in Microbiology Received: 13 February 2015 Accepted: 08 April 2015 Published: 29 April 2015
Citation: Chamoun R, Aliferis KA and Jabaji S (2015) Identification of signatory secondary metabolites during mycoparasitism of Rhizoctonia solani by Stachybotrys elegans. Front. Microbiol. 6:353. doi: 10.3389/fmicb.2015.00353
Stachybotrys elegans is able to parasitize the fungal plant pathogen Rhizoctonia solani AG-3 following a complex and intimate interaction, which, among others, includes the production of cell wall-degrading enzymes, intracellular colonization, and expression of pathogenic process encoding genes. However, information on the metabolome level is non-existent during mycoparasitism. Here, we performed a direct-infusion mass spectrometry (DIMS) metabolomics analysis using an LTQ Orbitrap analyzer in order to detect changes in the profiles of induced secondary metabolites of both partners during this mycoparasitic interaction 4 and 5 days following its establishment. The diketopiperazine(s) (DKPs) cyclo(S-Pro-S-Leu)/cyclo(S-Pro-S-Ile), ethyl 2-phenylacetate, and 3-nitro-4-hydroxybenzoic acid were detected as the primary response of Rhizoctonia 4 days following dual-culturing with Stachybotrys, whereas only the latter metabolite was up-regulated 1 day later. On the other hand, trichothecenes and atranones were mycoparasite-derived metabolites identified during mycoparasitism 4 and 5 days following dual-culturing. All the above secondary metabolites are known to exhibit bioactivity, including fungitoxicity, and represent key elements that determine the outcome of the interaction being studied. Results could be further exploited in programs for the evaluation of the bioactivity of these metabolites per se or their chemical analogs, and/or genetic engineering programs to obtain more efficient mycoparasite strains with improved efficacy and toxicological profiles. Keywords: metabolomics, mycoparasitism, mycotoxins, Rhizoctonia solani, direct-infusion mass spectrometry
Introduction Interactions between microbes encompass antagonistic, mycoparasitic, or competitive outcomes leading to the activation of complex regulatory mechanisms, which are regarded as a major route for the de novo biosynthesis of secondary metabolites (Schroeckh et al., 2009; Lorito et al., 2010; Brakhage and Schroeckh, 2011; Brakhage, 2013). Therefore, the study of the fungal secondary metabolites, implicated in such interactions, is expected to provide insights into key factors that determine their outcome. Mycoparasitism is a complex process when a fungus (mycoparasite) survives by using another fungus (host) as its source of nutrients. This involves a sequence of changes in the metabolism of both partners. Focusing on crop protection, mycoparasitism holds the premise of
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purchased from Fisher Scientific Company (Ottawa, ON, Canada).
becoming a valuable component of integrated pest management strategies (IPM) (Viterbo et al., 2007; John et al., 2010). To date, systematic research on mycoparasitism has been mainly performed on Trichoderma spp. (Lorito et al., 2010; Druzhinina et al., 2011; Mukherjee et al., 2013). Various other species such as, Coniothyrium minitans and Microsphaeropsis ochracea (Bitsadze et al., 2014), Aspergillus aculeatus (Hu et al., 2013), and Stachybotrys elegans (Chamoun et al., 2013), have shown potential as mycoparasites of important plant pathogens. S. elegans parasitizes the soil-borne fungal pathogen Rhizoctonia solani. During this intimate interaction, S. elegans cell walldegrading enzymes (Taylor et al., 2002; Morissette et al., 2003) and mycoparasitism-associated genes involved in pathogenic processes (Morissette et al., 2008) are expressed. In response to mycoparasitism, transcript levels of a R. solani pyridoxal reductase-encoding gene, whose role in reactive oxygen species (ROS) quenching is established, are elevated (Chamoun and Jabaji, 2011). In contrast to the wide range of applications of metabolomics in plant, animal, and human-related research (Griffin, 2006; Hall, 2006; Spratlin et al., 2009; Aliferis and Jabaji, 2011), microbial metabolomics is still in its infancy. Studies investigating metabolic aspects of microbes have mainly focused on fungal classification (Smedsgaard et al., 2004; Aliferis et al., 2013), metabolic profiling of antagonistic interactions (Tsitsigiannis et al., 2005; Rodriguez Estrada et al., 2011; Combès et al., 2012; Jonkers et al., 2012; Bertrand et al., 2013) or interactions between primary and secondary fungal colonizers of wood (Peiris et al., 2008). Nonetheless, metabolomics has not been yet applied for the study of mycoparasitic interactions. The main task of the present research is to dissect the undergoing changes in the profile of the secondary bioactive metabolites of both fungal partners during mycoparasitism. This could provide valuable insights into the main factors that determine its outcome. Here, a metabolic profiling strategy was applied performing direct infusion mass spectrometry (DIMS) analysis using a linear trap quadrupole (LTQ) Orbitrap Classic analyzer. Moreover, because metabolite identification represents a bottleneck for fungal metabolomics, (El-Elimat et al., 2013), here it was performed by using a targeted in-house built speciesspecific metabolic database for Rhizoctonia and Stachybotrys secondary metabolites. Following dual-culturing, the metabolic profiles of secondary metabolites of R. solani and S. elegans, were recorded. Such information could be further exploited in crop protection for the production or synthesis of new antifungal agents or for designing selection and genetic engineering programs to obtain more efficient strains of the mycoparasite with improved toxicological profiles.
Biological Material Starter cultures of the mycoparasite Stachybotrys elegans (Pidoplichko) W. Gams (ATCC 18825) and the pathogen Rhizoctonia solani AG-3 (ATCC 10183) were revived from precolonized oat kernels on 1% potato dextrose agar (PDA; Difco Laboratories, Michigan, USA) and incubated at 24◦ C for 7 and 5 days, respectively. Induction and collection of S. elegans conidia were performed as previously described (Chamoun and Jabaji, 2011).
Establishment of Mycoparasitic Interaction Dual-culturing of S. elegans and R. solani was conducted in 9 cm Petri plates containing 20 mL of minimal synthetic medium (MSMA) composed (g L−1 ) of: MgSO4 .7H2 O, 0.2; K2 HPO4 , 0.9; KCl, 0.2; FeSO4. 7H2 O, 0.002; MnSO4 , 0.002; ZnSO4 , 0.002; NaNO3 , 1.0; biotin, 10 mg; gellan gum, 1% (composed of glucose, glucuronic acid and rhamnose in the molar ratio of 2:1:1) (Phytagel, Sigma, St. Louis, USA). Agar plugs (8 mm) of a 5-day old R. solani culture were grown on MSMA for 48 h and then sprayed with 100 µL of a suspension of S. elegans conidia (106 mL−1 water) using a Badger 350 air brush and MC-80 mini air compressor calibrated at 1 kg cm−2 . The control treatments consisted of spraying 100 µL of S. elegans conidia on non-inoculated MSMA plates and R. solani-inoculated MSMA plates sprayed with sterile distilled water. Additionally, a negative control representing the MSMA medium was used to determine compounds of non-biological origin. All culture plates were incubated at 24◦ C for 4 or 5 days following dual and pure strain cultivation. These time points were chosen based on a priori knowledge to capture the infection and colonization of R. solani hyphal cells by S. elegans (Chamoun and Jabaji, 2011). Five replications were performed per treatment.
Optical Microscopy To associate the metabolic changes with the progress of the mycoparasitic process, agar pieces (5 × 5 mm) from interaction zones of dual-culture plates and from pure cultures of both fungal partners were collected in a time course. Sections from interacting zones were stained with lactophenol blue or water and viewed under an optical microscope. Presence of hyphal coils, penetration pegs and intracellular colonization of the pathogen was digitally documented with the Moticam 2300 digital camera (GENEQ Inc. QC, Canada).
Sampling, Quenching, and Metabolite Extraction Four plugs (8 mm in diameter × 7 mm in height) were collected from the interaction zones of dual-cultures, pure cultures of each fungal partner after 4 or 5 days of cultivation and from the negative control (MSMA) plates. Plugs were placed in glass autosampler screw thread vials (2 mL, Fisher Scientific, ON, Canada). Quenching was instantly performed by adding twice 2 mL of liquid N2 , and samples were stored at -80◦ C until further processing. Extraction was performed as previously described (Aliferis et al., 2014). Briefly, 1 mL of a mixture of methanolethyl acetate (50:50, v/v) was added to the vials, followed
Materials and Methods Chemicals and Reagents All chemicals used for metabolite extraction and sample preparation for DIMS analysis were of the highest commercially available purity. Methanol, ethyl acetate, formic acid, ammonium R grade), and water (HPLC grade) were acetate (Optima
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samples (MSMA) and also detected in the biological samples were removed and were excluded from further analysis. The obtained aligned matrix was then exported to Microsoft Excel for further processing. Finally, the matrix composed of identified secondary metabolites detected in ESI+ and ESI− was exported to the SIMCA-P+ v.12.0.1 software (Umetrics, MKS Instruments Inc., MA, USA) for multivariate analysis (Aliferis and Jabaji, 2012). The discovery of biomarker-ions was based on partial least squares-discriminant analysis (PLS-DA) regression coefficients (P < 0.05). Based on the variability in the model parameters encountered in the different cross-validation cycles, standard errors were calculated with 95% confidence interval using Jack-knifing (Efron and Gong, 1983).
by sonication for 25 min. Samples were further extracted for 2 h under continuous agitation (250 rpm) at 25◦ C and filtered through 0.2-µm filters (Millex-FG; Millipore, MA, USA). The volume of samples was adjusted to 1 mL and subsequently divided into two equal portions (0.5 mL) for DIMS analyses in positive (ESI+ ) and negative (ESI− ) electrospray modes. Finally, extracts were dried using a Labconco CentriVap refrigerated vacuum concentrator equipped with a cold trap (Labconco, MO, USA).
Direct Infusion Mass Spectrometry (DIMS) and DIMS/MS Analysis For DIMS and DIMS/MS analyses, an LTQ Orbitrap MS Classic (Thermo Scientific, CA, USA) was used acquiring in the ESI+ or ESI− modes (Aliferis et al., 2014). All experimental events were controlled by the software Xcalibur v.2.2 (Thermo Scientific). The analyzer was equipped with a heated electrospray ionization probe (HESI-II, Thermo Scientific), a quadrupole linear ion trap, and an Accela pump (Thermo Scientific). For analysis in ESI+ and ESI− , 100 µL of a mixture of methanol/formic acid (0.2% v/v) (50–50, v/v) or methanol/ammonium acetate (4 mM) was added to the dried samples, respectively. Extracts were then transferred to glass microinserters (150 µL), which were consecutively placed into 2 mL glass autosampler vials. Samples (10 µL) were injected at a flow rate of 10 µL min−1 using a 100 µL syringe (Hamilton, NV, USA). Full scan mass spectra were acquired in the range between 50 and 1200 Da at a rate of 0.6 scans/s and a mass resolution of 60,000 at 400 m/z. The source and capillary voltages were set to 3.2 kV and 5.0 V for ESI+ and 4.0 kV and −35 V for ESI− , respectively. The capillary temperature for both modes was set to 275◦ C. Sheath gas flow was set to 10 (ESI+ ), and 20 (ESI− ) whereas no auxiliary and sweep gases were used. For selected samples, MS/MS spectra were recorded using previously described settings (Aliferis et al., 2014).
Metabolite Identification and Assignment of Their Origin during Mycoparasitism The identification of metabolites was performed following a biologically-driven approach performing searches against the targeted in-house species-specific metabolic databases for Rhizoctonia and Stachybotrys. The libraries were constructed acquiring information from the literature and publicly available databases such as, KNApSAcK (http://kanaya.naist. jp/KNApSAcK/) and PubChem (http://pubchem.ncbi.nlm.nih. gov/). Identification of metabolites was based on mass accuracy (
